Cryogenic capacitive bolometer

ABSTRACT

A capacitive bolometer suitable for measuring electromagnetic radiation when operated in the cryogenic temperature region. The dielectric material used in the bolometer element is one having a negative or nearly zero temperature coefficient of self-heating. The dielectric member of the element may be made from a composition in which strontium titanate, SrTiO3, has been controllably crystallized in a glass matrix. A plurality of elongated electrically conductive members are disposed adjacent each of two substantially parallel surfaces of the dielectric member in a substantially parallel arrangement and spaced from one another. The electrically conductive members on one surface of the dielectric member are disposed in opposition to the plurality of members on the other surface and arranged substantially perpendicular thereto so that at least one elongated member on one surface of the dielectric layer crosses at least one of the elongated members disposed adjacent the opposite surface of the dielectric layer.

United States Patent 1191 Lawless et al.

[ CRYOGENIC CAPACITIVE BOLOMETER [75] Inventors: William N. Lawless; Brent M.

Wedding, both of Corning, NY.

[73] Assignee: Corning Glass Works, Corning,

[22] Filed: May 15, 1973 [21] Appl. No 360,412

[52] US. Cl 317/247, 317/246, 317/256, 317/258 [51] Int. Cl ll-l0lg 7/04 [58] Field of Search 317/246, 247, 256, 258, 317/261,'242

[56] References Cited UNITED STATES PATENTS 3,208,037 8/1965 Botwin 317/246 3.453.432 7/1969 McHenry 317/247 OTHER PUBLICATIONS Albrecht, Notes On Capacitance Bolometers in Proceedings in [RE Australia, 4-57, pp. 128-129.

Primary Examiner-E. A. Goldberg Attorney, Agent, or Firm-Walter S. Zebrowski June 4,1974

[5 7] ABSTRACT A capacitive bolometer suitable for measuring electromagnetic radiation when operated in the cryogenic temperature region. The dielectric material used in thebolometer element is one having a negative or nearly zero temperature coefficient of self-heating. The dielectric member of the element may be made from a composition in which strontium titanate,

SrTiO has been controllably crystallized in a glass matrix. A plurality of elongated electrically conductive members are disposed adjacent each of two substantially parallel surfaces of the dielectric member in a substantially parallel arrangement and spaced from one another. The electrically conductive members on one surface of the dielectric member are disposed in opposition to the plurality of members on the other surface and arranged substantially perpendicular thereto so that at least one elongated member on one surface of the dielectric layer crosses at least one of the elongated members disposed adjacent the opposite surface of the dielectric layer.

12Claims, 7 Drawing Figures PATENTEUJUNY 41914 SHEET 2 0F 2 v? 2 3. -55 m6 mwmwm 9 m llll lllllllll llllll ll yr 5 P m5... goiwwwm o m O M6 M6 1 CRYOGENIC CAPACITIVE BOLOMETER I BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a cryogenic capacitive bolometer having high detectivity within at least the temperature range of O.3K to 20K. More specifically, this invention relates to a cryogenic capacitive bolometer wherein the dielectric material has a negative or nearly zero temperature coefiicient of self-heating, that i is equal to or less than about 0.0l7/K, permitting the responsivity and detectivity of the device to be maximized.

Background of the Invention A variety of bolometers have been known in the prior art including resistance bolometers, thermocouple bolometers, pneumatic bolometers, semiconducting bolometers, superconducting bolometers, and capacitance bolometers. All of the heretofore known bolometers have had serious disadvantages. For example, the sensitivity of the AC metallic resistance bolometer has been limited by the low temperature coefficient of resistance obtainable with metals and a low resistance of the metallic elements. Therefore, such bolometers cannot be well matched to the input impedance of an AC amplifier. Bolometer elements of higher resistance have been made by sputtering metal films, however, such bolometer elements were also not found to have a resistance suitable for acceptable sensitivity of the device. All available resistance bolometers are limited by Johnson noise, which is determined by the real part of the electrical impedance. In semiconductor bolometers, another type of noise, called current noise, has to be considered. Furthermore, such semiconducting bolometers must be operated at very low temperatures requiring an elaborate refrigerator to get good results.

It has been found that bolometers have heretofore notonly required additional auxiliary equipment but provided little, if any, advantages over other means for measuring electromagnetic radiation. In addition, superior bolometers have been difficult and expensive to manufacture as well as fragile thereby preventing their coming into common use. Still further, the physical parameters of the materials from which bolometer elements have been made as well as the materials themselves have prevented the optimization of performance of bolometers.

SUMMARY or THE INVENTION Therefore, it is the object of this invention to provide a device sensitive to electromagnetic radiation suitable for use as a cryogenic capacitive bolometer, and which overcomes the aforementioned disadvantages.

Briefly, this invention comprises a device with a dielectric member or layer having two substantially parallel surfaces formed of a material having a temperature coefficient of self-heating equal to or less than about 0.017. The temperature coefficient of selfheating, a, is a function of the loss tangent and the real part of the dielectric constant substantially in accordance with the equation a=- de /HIT) 6tan6/tan8- 6 7) wherein tan8 is the. loss tangent, e is the real part of the dielectric constant, and

2.: T is the temperature of the dielectric member or layer.

To one of the parallel surfaces of such a dielectric member is applied a first'plurality ofelectrically interconnected, elongated, electrically conductive members disposed in a substantially parallel arrangement and spaced from one another. On the opposite parallel surface of the dielectric member in opposition to said first plurality of members, a second plurality ofelectrically interconnected, elongated, electrically conductive members are disposed in a substantially parallel arrangement and spaced from one another. The second plurality of members are arranged substantially perpendicularly to the first plurality of members such that at least one elongated member on one surface of the dielectric member crosses at least one of the elongated members disposed on the. opposite surface of the dielectric member.

The dielectric member or layer may be-formed of glass from which strontium titanate in combination with other dopants has been controllably crystallized. Input and output terminals are respectively connected to the first and second plurality of members and the detector or the sensing area of the device may thereafter be appropriately blackened.

" Additional objects, features andadvantages of the present invention will become apparent to those skilled in the art from thefollowing detailed description and the attached drawings on which, by way of example,

only the preferred embodiments illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustration partly in section of the bolometer of the present invention and equipment associated therewith.

FIG. 2 is an oblique view of a bolometer element of the present invention.

FIG. 3 is an illustration of another embodiment of a bolometer element of the-present invention.

FIG. 4 is a schematic diagram. of the equivalent circuit of the cryogenic capacitive bolometer of the present invention. 7

FIG. 5 is a cross-sectional illustration'of still another embodiment of a bolometer element of the present im vention. I

FIG. 6 is a graphical representation showing the relationship between the reservoir temperature in degrees Kelvin and a function of the responsivity (r) of the bolometer element of this invention.

FIG. 7 is a graphical representation showing the relationships between reservoir temperature in degrees Kelvin and both a function of the noise equivalent power (NEP)- and the detectivity (D*) of the device of this invention.

DETAILED DESCRIPTION OF THE INVENTION It is to be noted that FIGS. 1 through 5 of the drawings are illustrative and symbolic of the invention, and there is no intention to indicate scale, relative proportions, or materials of the elements shown therein.

Referring now to FIG. 1, a capacitive cryogenic bolometer system is shown diagrammatically. Bolometer element 10 is shown formed of a dielectric layer 12 having two flat surfaces. The material of dielectric layer 12 has a negative or nearly zero temperature coefficientof self-heating, that is a s 0.0l7/K, and will of this invention are disposed adjacent the other flat surface of dielectric,

layer 12. Members 18 are also electrically interconnected by connecting member 20 and are arranged ina substantially parallel arrangement spaced from one another. Members 18 are disposed in opposition to members 14 and are arranged substantially perpendicularly tomembers 14 whereby at least one elongated member'l8 crosses at least one elongated member 14 as shown in FIG. 2. Elongated members 14 and 18 as well as connecting members 16 and 20 may be disposed adjacent the flat surfaces of dielectric layer 12 in any manner known in the art such, for example, as by applying an electrically conductive frit, paste, or the like and thereafter suitably drying and firing it as required; or by adhering a metallic foil to the flat surfaces of the dielectric layer. A metallic foil, for example, may be applied and adhered after it is patterned or may be applied in' sheet form and thereafter patterned. Also, the elongated and connecting members may be chemically or mechanically deposited on the flat surfaces of dielectric'layer 12in various ways well known in the art such as sputtering, vapor deposition, and the like.

Thereafter, black surface 22, formed of lampblack or any other known blackening material, is applied over one of the plurality of elongated electrically conductive members by first brushing-on a suitable binder, such as varnish suitable for low temperatures and high vacuum, and then sprinkling lampblack thereover until a substantially continuous layer thereof is formed. The binder is then dried adhering the layer of lampblack. One familiar with the art can readily select a varnish suitable for the present purposes. Any other method known in the art for forming a black surface for low temperatures and high vacuum is suitable for forming black surface 22 of the present invention.

Bolometer element'l0 is then attached to thennal reservoir 24 by means of thermal links 26. As will hereinafter be described in detail, the material of thermal links 26 must possess certain physical properties. Thermal links 26 are shown in FIG. 1 as separate members attached between dielectric layer 12 of bolometer element l and thermal reservoir 24. The thermal links may, however, be integral with the dielectric layer as shown in FIG. 3 wherein dielectric layer 28 is shown sufficiently extended beyond the elongated electrically conductive members 30 and 32 disposed on opposite surfaces of layer 28 as hereinbefore described. In such an embodiment, the bolometer element 34 with integral thermal links is attached to a thermal reservoir at the peripheral edge 36 of dielectric layer 28.

Referring again to FIG. 1, the plurality of electrically conductive members 14 and 18 are shown connected across constant amplitude current generator 38.

The operation of the cryogenic capacitive bolometer of the present invention and the underlying theory is described as follows. Referring to FIG. I, electromagnetic radiation 40 from a suitable source, not shown, is chopped by a chopper 42 at a suitable frequency whereby the resulting noise band width is narrowed by detecting the bolometer output voltage with a phase sensitive means. A chopper suitable for the present purposes may be a rotating sector wheel having slotted openings disposed in such a manner as to periodically interrupt a beam of electromagnetic radiation into a train of pulses having an angular frequency of w,- (sec One familiar with the art can readily select a suitable chopper.

Bolometer element 10 is thermally coupled to thermal reservoir 24 by means of thermal links 26 and is driven with a constant bias current from generator 38 resulting in self-heating of the bolometer element. The heat balance equation under these conditions is where G is the specific heat capacity (W-secdeg), T is the temperature of the bolometer element (K),

AT HI T-T e'rW/G(l w 7- where AT is the temperature difference,

T is the operating temperature ("K), and

r is the thermal time constant (sec). As will be hereinafter further explained, it may be assumed that the heat due to self-heating, Q, caused by the constant bias current, is linearly dependent on the temperature of the bolometer element, T, in accordance with the equation where Q, is the self-heating constant (watts), and a is the temperature coefficient of self-heating g The thermal time constant is defined by the equation and the zero-radiation or operating temperature, T is expressed by the equation The steady state equation (2) shows that the harmonic radiation heating causes a harmonic temperature variation about the operating temperature while equation 4 demonstrates that the thermal time constant is a function of self-heating. This can be demonstrated by the situation where the temperature coefficient of self heating is greater than 0, (a wherein self-heating is greater at the radiation induced temperature than at the operating temperature. In such a situation, the bolometer element cools more slowly to the operating temperature than in a situation where no self-heating exists.

The equivalent circuit for the cryogenic capacitive bolometer is shown in FIG. 4 wherein the bolometer element is represented by the pure capacitance C and the equivalent series resistance R to account for AC losses. R is defined as being equal to tanS/wC. The bolometer element is driven with a constant amplitude AC current, le and the voltage V across the element is obtained in accordance with the equation V 1(1 tan 8)" /wC The heat causing self-heating of the bolometer element under constant bias current is determined in accordance with the equation from which the temperature coefficient of self-heating inequation 3 can be identified as It is therefore seen that the temperature coefficient of self-heating, of which the thermal time constant is a function as seen from equation 4, is a function of the temperature coefficients of both the in-phase loss tangent and out-of-phase capacitance components. Accordingly, the assumption of linear self-heating made in equation 3 is justified for the situation where both 6 CIC GT and 6 tan8/tan86 T are not strongly temperature dependent in the temperature range of interest.

It has been found that the materials which have either a negative or nearly zero temperature coefficient of self-heating, that is a sq) 1 7/ K,' a r e most suitable for fabricating the dielectric member of the bolometer of the present invention. Some examples of materials suitable for fabricating the dielectric member are set out in Table I wherein the constituents are given in weight percentages on an oxide basis.

TABLE I Material Type SrTiO SiO, A1 0, Nb- .O NiO A 69 20 10 1 B 69 22 9 C 72 23 5 D 23 5 E 70 22 6 F 63 25 l 5 G 68 25 5 2 H 73 22 4 Material Type SnO, ZnO SrF, Ta,O

D E l l F 7 G H l It has also been found that for materials which are suitable for fabricating the dielectric member of the present invention the loss tangent, tan8, is relatively small as compared to unity. Therefore, the expressions 1 3tan 6 and l tan fi are, for all practical purposes, equal to 1. Accordingly, equations 4, .7, 8, and 9 can be simplified respectively as follows:

e is the real part of the dielectric constant, equation 13 can be expressed as The responsivity and detectivity are the two most important parameters of a bolometer. The responsivity, r, of a bolometer is defined as the ratio of the output voltage variation to p the incident radiation power, r

i AV/ W. Using equations 2 and ll r I( 86/6 6 T)eR'r/tan6G( l (0 7 The responsivity is maximized by an appropriatey chosen value of the bias current amplitude, l, which is determined as follows. The operating temperature, T is determined by I in accordance with equation 12. The specific heat capacity, G, of insulating materials is known from the Debye Theory to be proportional to the third power of the absolute temperature in the cryogenic temperature region and will therefore depend on the operating temperature. For convenience, the operating temperature is made the independent variable in where x is the fractional temperature increase of the operating temperature above the reservoir temperature, T The bolometer dimensions can be selected to adjust the thermal conductance so that the thermal time constant has a fixed value.

In determining the maximum responsivity, the specific heat capacity is expressed by where.

G (T,,) is the specific heat capacity at T,

(W-sec-deg). In view of equation 17, equation 12 becomes FR fT x, therefore, combining equations and I8 F4 (To)(l X) /T(l aT x) and from equation 12 (T..)( +x)Tox/1R(1 1T x)] Using these equations, equation 16 becomes The responsivity, r, will be maximum when dr/dx 0, which occurs when The detectivity of the bolometer involves noise considerations. It is convenient to consider the bolometer noise in terms of noise equivalent power, NEP, which is defined as the radiant input power that is just equal to the dark current noise. For the purposes of the present invention, three inherent noise contributions will be considered. In the following equation, the first term is the square of the Johnson noise voltage across the element divided by the square of the responsivity (Johnson noise power), the second term is the square of the temperature noise power, and the third term is the square of the noise power from random fluctuations of the emission of radiation from the element:

where A is the detector area (cm k is the Boltzman constant (1.38 X l0 J'deg Af is the bandwith of chopped radiation (Hz),

0' is the Stefan-Boltzman constant (5.67 X

NEP is the noise equivalent power (W-Hz).

Combiningequations l7, I8, 20, 21, and 24 gives the noise equivalent power at maximum responsivity represented by where C,,(T is the volumetric specific heat at T,,(W'seccm 'de'g"), and

d is the bolometer element thickness (cm.). The detectivity, D*,.in (cm-w' -Hz of the bolometeris defined as the reciprocal of the noise equivalent power corrected for the detector area of the bolometer element and is represented by D* A /NEP Examination of equations 24, 25, and 26 shows that, for the case where the Johnson noise is the dominant noise contribution, the detectivity, D*, is also maximized by the bias current, I. It has been found that the maximum values of-r* and D* occur at practically the same value of 1*.

FIG. 5 illustrates still another embodiment of a bolo'meter of the present invention. The material of dielectric element 44 has a temperature coeffient as heretofore described and element 44 is formed in a curved shape with substantially parallel surfaces 46 and 48. A first plurality of elongated electrically conductive members 50 are disposed adjacent surface 46 in a substantially parallel arrangement spaced from one another. A second plurality of elongated electrically conductive members 52 are disposed adjacent surface 48 in a substantially parallel arrangement and spaced from one another, but disposed in opposition to members 50,

Example I Aspecific example of the present invention is illustrated by the'following. A bolometer element such as that illustrated in .FIG. 3 is formed from material type F shown in Table l. The-material is formed in the shape of a disc having a thickness of 0.0 10 cm. and an outside diameter of 'l.27 cm.'A pattern of electrically conductive members is thereafter applied, to each flatsurface of the discby photolithographic means. Each of these patterns is circular having a diameter of about 0.485 cm. and comprises twenty parallel electrically conductive members; The members on one surface are disposed in opposition to the members on' the other surface and are arranged substantially perpendicularly thereto so that members on 'one'surface cross at least some of the members on the opposite surface. The width of the members is approximately 5 X inches or about one-half mil. The spacing between the parallel conductive members is about-9.6 X 10 inches. .With the real partof thedielectric constant, 6, being 135 for the material selected, the resulting capacitance of the device is 10 picofarads. The thermal time constant is 10' seconds. a A black surface is applied over the grid on one of the flat surfaces of the disc by bonding lampblack with a small amount of thin varnishas heretoforedescribed. Thereafter,fthe bolometer element is attachedto a reservoir having an operating temperature of 4.2K.'A constant amplitude current generator is then connected to each of the grids'of the electrically conducting members'on the flat surfaces of the disc and a field strength of 86 KV/cm. at lKHz is applied thereto. The optimum responsivity of such a device is 2.2 X 10 V-W", the noiseequivalent power is 1.0 X l0 W-Hz" ,and the optimum detectivity is 7.6 X 10 cm'WHz. FIGS. 6 and 7 illustrate the relationship between the reservoir temperature and the maximized values of a function of responsivity, a function of NEP, and the detectivity of bolometer elements of the present invention, such as that described in the preceding example. I t 7 Although the present invention has been described with respect tospecific details of certain embodiments thereof, it is not intended that such details be limita? v T is the temperature of the dielectric layer,

' a first plurality of electricallyinterconnected, elongated, electrically conductive members disposed adjacent one of said substantially parallel surfaces of said dielectric member in a substantially parallel arrangement and spaced from one another,

a second plurality of electrically interconnected,

elongated, electrically conductive members disposed adjacent the other of said substantially'parallel surfaces of said dielectric member in a substantially parallel arrangement and spacedfrom one another, said second plurality of members also being disposed in opposition to said first plurality of members and arranged substantially perpendicularly to said first plurality of members such that at least 'one elongated member crosses at least one of theelongated members disposed adjacent the opposite surface of said dielectric member, and

terminablemeans connected to said first and second plurality of members, the capacitance of said bolometer being a determinable and measurable function of incident electromagnetic radiation.

' 2. The cryogenic capacitive bolometer of claim lfurther comprising a thermal reservoir.

3. 'The'cryogenic fapacitive-bolometer of claim 1 wherein said dielectric member isa flatlayer and said parallel surfaces are opposite flat surfaces of said layer.

tions onthe present invention except insofar as is set forth in the following claims.

. arr/e a T) a tan6/tan8- a T) wherein I 'terial as said dielectricmember.

4. The cryogenic capacitive bolometer of claim 2 further comprising thermal links disposed intermediate. said dielectricmember and said thermal reservoir.

5. The cryogeniccapacitive bolometer of claim 4 wherein said thermal links are formed of the same ma-' 6. The cryogenic capacitive bolometer of claim 5 wherein said thermal links and said dielectric member are formed as a single unit. 7. The cryogenic capacitive bolometer of claim 1 further comprising a black surface disposed over said first plurality of electrically I interconnected, elongated, electrically conductive members.

8. The cryogenic capacitive bolometer of claim 7 further comprising input and output terminal leads respectively connected to said first and second pluralityof electrically conductive members. 1

9. The cryogenic capacitivebolometer of claim 1 wherein said dielectric member is formed of glass from which strontium 'titanate in combination with other dopants has been controllablycrystallized. I 10. The cryogenic capacitive bolometer of claim 1 wherein said dielectric member is formed of material consisting essentially of 63 percent SrTiO 25 percent SiO 5 percent A1 0,, and 7 percent Ta O by weight, from which at least a portion of said strontium titanate has been controllably crystallized.

11. The cryogenic capacitive bolometer of claim 1 wherein said dielectric member is curved having a concave detector surface.

12. The cryogeniccapacitive bolometer of claim I further comprising means for simultaneously maximiz ing the responsivity and detectivity. thereof.

- at as 

2. The cryogenic capacitive bolometer of claim 1 further comprising a thermal reservoir.
 3. The cryogenic capacitive bolometer of claim 1 wherein said dielectric member is a flat layer and said parallel surfaces are opposite flat surfaces of said layer.
 4. The cryogenic capacitive bolometer of claim 2 further comprising thermal links disposed intermediate said dielectric member and said thermal reservoir.
 5. The cryogenic capacitive bolometer of claim 4 wherein said thermal links are formed of the same material as said dielectric member.
 6. The cryogenic capacitive bolometer of claim 5 wherein said thermal links and said dielectric member are formed as a single unit.
 7. The cryogenic capacitive bolometer of claim 1 further comprising a black surface disposed over said first plurality of electrically interconnected, elongated, electrically conductive members.
 8. The cryogenic capacitive bolometer of claim 7 further comprising input and output terminal leads respectively connected to said first and second plurality of electrically conductive members.
 9. The cryogenic capacitive bolometer of claim 1 wherein said dielectric member is formed of glass from which strontium titanate in combInation with other dopants has been controllably crystallized.
 10. The cryogenic capacitive bolometer of claim 1 wherein said dielectric member is formed of material consisting essentially of 63 percent SrTiO3, 25 percent SiO2, 5 percent Al2O3, and 7 percent Ta2O5 by weight, from which at least a portion of said strontium titanate has been controllably crystallized.
 11. The cryogenic capacitive bolometer of claim 1 wherein said dielectric member is curved having a concave detector surface.
 12. The cryogenic capacitive bolometer of claim 1 further comprising means for simultaneously maximizing the responsivity and detectivity thereof. 